Distiller employing cyclical evaporation-surface wetting

ABSTRACT

A distillation unit ( 10 ) employs a rotary heat exchanger ( 32 ) forming a multiplicity of evaporation chambers ( 56 ) into which a liquid to be purified is sprayed for evaporation. Spray arms ( 58 ) spray at a steady rate into all of the evaporation chambers ( 56 ) simultaneously but not at a rate that is adequate to maintain the wetting required for efficient transfer of heat to the liquid. A scanning sprayer ( 140 ) supplements this steady spray with spray from nozzles ( 142  and  144 ) into only a few of the evaporation chambers at a time, visiting all of them cyclically. The overall rate of spray from the two sources thus combined to spray the chamber cyclically maintains proper wetting even though on average it is lower than the rate that would be required of a constant-rate spray into all of the evaporation chambers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to commonly assigned U.S. Pat. No.6,689,251 to William H. Zebuhr entitled Cycled-Concentration Distiller,U.S. patent application Ser. No. 09/765,260 of William H. Zebuhrentitled Distiller Employing Separate Condensate and ConcentrateHeat-Exchange Paths, abandoned U.S. patent application Ser. No.09/765,261 of William H. Zebuhr entitled Rotary Evaporator EmployingSelf-Driven Recirculation, and U.S. patent application Ser. No.09/765,475 of William H. Zebuhr entitled Distiller EmployingRecirculation-Flow Filter Flushing, all of which were filed on Jan. 18,2001, and are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to distillation. It has particular,but not exclusive, application to using rotary heat exchangers to purifywater by distillation.

2. Background Information

One of the most effective techniques for purifying water is to distillit. In distillation, the water to be purified is heated to the point atwhich it evaporates, and the resultant vapor is then condensed. Sincethe vapor leaves almost all impurities behind in the input, feed water,the condensate that results is typically of a purity much higher in mostrespects than the output of most competing purification technologies.

One of the distillation approaches to which the invention to bedescribed below may be applied employs a rotary heat exchanger. Water tobe purified is introduced to one, evaporation set of heat-exchangesurfaces, from which the liquid absorbs heat and evaporates. Theresultant water vapor is then typically compressed and brought intocontact with another, condensation set of heat-exchange surfaces thatare in thermal communication with the set of evaporation heat-exchangesurfaces. Since the water vapor on the condensation side is undergreater vapor pressure than the water on the evaporation side, vaporthat condenses on the condensation side will be hotter than theevaporating liquid on the evaporation side, and its heat ofevaporization will therefore flow to the evaporation side: the systemreclaims the heat of evaporization used to remove the relatively purevapor from the contaminated liquid. To minimize the insulating effectsto which a condensation film on the condensation surfaces would tend tocontribute, a rotary heat exchanger's heat-exchange surfaces rotaterapidly, so the condensate experiences high centrifugal force and istherefore removed rapidly from the condensation surfaces.

This removal of liquid from the condensation-side heat-exchange surfacesis important, because a significant drawback of using distillation forwater purification is the energy cost that it exacts. That cost tends tobe greater when the temperature difference between the rotary heatexchanger's evaporation and condensation sides is relatively great. Onthe other end, a low temperature difference tends to result in a lowerrate of heat exchange, and this then necessitates a greaterheat-exchange area for a given volume rate of distillation. Such anadditional heat-exchange-surface area exacts its own cost penalties notonly in initial equipment cost but also in the power needed to operatethe unit. The reason why rapid condensate removal tends to amelioratethe energy-cost problem is that reduction of the condensate film'sinsulating effects tends to increase the heat-exchange rate for a giventemperature difference.

The rotary heat exchanger's centrifugal force also tends to reduce thewater-film thickness on the evaporation side and thereby further benefitheat-exchange efficiency. Of course, introducing liquid to theevaporation side at too great a rate will compromise the centrifugalforce's beneficial effect on heat transfer, so evaporator efficiency isbest served by keeping the rate of feed-water introduction relativelylow. Unfortunately, too low a rate of feed-water introduction iscounterproductive; it allows surface tension to defeat proper surfacewetting and thus heat transfer to the liquid.

SUMMARY OF THE INVENTION

But I have recognized that heat-exchanger efficiency can be improved byemploying a technique that keeps the evaporator surfaces substantiallywetted but uses an average rate of liquid feed substantially lower thanthe steady-state rate required to maintain proper wetting. In accordancewith my invention, the rate at which the evaporator-side heat-exchangesurfaces are irrigated so varies as repeatedly to reach a peakirrigation rate that is at least twice its average rate. Preferably,that average rate is less than half the steady-state rate required tomaintain proper wetting, while the peak rate preferably exceeds thatsteady-state rate. Even though the average rate is low, the repeatedincreases to such a peak rate can prevent those surfaces from dewetting.The result is a significantly greater heat-exchange rate, and less powerconsumption, than in a similar system employing the minimum steady-staterate required to maintain wetting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a front isometric view of a distillation unit that employs thepresent invention's teachings;

FIG. 2 is a cross-sectional view taken through the distillation unit;

FIG. 3 is a plan view of one of the heat-exchange plates employed in thedistillation unit's rotary heat exchanger;

FIG. 4 is a cross-sectional view through two such plates taken at line4—4 of FIG. 3;

FIG. 5 is a diagram of the fluid flow through the rotary heatexchanger's evaporation and condensation chambers;

FIG. 6 is a broken-away perspective view of the distillation unit'scompressor;

FIG. 7 is a broken-away cross-sectional view of one side of thecompressor and the rotary heat exchanger's upper portion showing thefluid-flow paths between them;

FIG. 8 is schematic diagram of the distillation unit's fluid circuit;

FIG. 9 is a perspective view of the vapor-chamber base, main scooptubes, and irrigation arms that the distillation unit employs;

FIG. 10 is a plan view of the elements that FIG. 9 depicts;

FIG. 11 is a cross-sectional view taken at line 11—11 of FIG. 10;

FIG. 12 is a cross-sectional view taken at line 12—12 of FIG. 10;

FIG. 13 is a cross-sectional view of one of the spray arms, taken atline 13—13 of FIG. 12;

FIG. 14 is a broken-away perspective view of the distillation unit'stransfer valve and related elements;

FIG. 15 is a broken-away perspective view of the distillation unit'stransfer pump;

FIG. 16 is a broken-away isometric view of the distillation unit'sfilter assembly;

FIG. 17 is a further broken-away perspective view of the transfer valveillustrating the valve crank and its actuator in particular;

FIG. 18 is a view similar to FIG. 12, but showing the transfer valve inits elevated position;

FIG. 19 is an isometric view of one of the distillation unit'scounterflow-heat-exchanger modules; and

FIG. 20 is a cross-sectional view of that heat-exchanger module.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1 is an exterior isometric view of a distillation unit in which thepresent invention's heat-exchanger-irrigation approach can be employed.In general, the distillation unit 10 includes a feed inlet 12 throughwhich the unit draws a feed liquid to be purified, typically watercontaining some contamination. The unit 10 purifies the water, producinga pure condensate at a condensate outlet 14. The volume rate ofcondensate produced by the unit 10 will in most cases be only slightlyless than that of the feed liquid entering inlet 12, nearly all theremainder being a small stream of concentrated impurities dischargedthrough a concentrate outlet 16. The unit also may include asafety-drain outlet 18. The illustrated unit is powered by electricity,and it may be remotely controlled or monitored. For this reason,electrical cables 20 are also provided. In the illustrated embodiment,the distillation unit 10 is intended for high-efficiency use, so itincludes an insulating housing 22. But the present invention's teachingsare applicable to a wide range of heat-exchanger applications, not allof which would typically employ such a housing.

FIG. 2 is a simplified cross-sectional view of the distillation unit. Itdepicts the housing 22 as having a single-layer wall 24. In single-layerarrangements, the wall is preferably made of low-thermal-conductivitymaterial. Alternatively, it may be a double-layer structure in which thelayers are separated by insulating space.

The present invention is an advantageous way to supply feed liquid tothe unit's heat exchanger 32. While the present invention's teachingscan be employed to feed a wide variety of heat exchangers, the drawingsillustrate a particular type of rotary heat exchanger for the sake ofconcreteness. As will be explained in more detail directly, theillustrated embodiment's rotary heat exchanger is essentially a group ofstacked plates, one plate 34 of which will be described in more detailin connection with subsequent drawings. That heat exchanger 32 is partof an assembly that rotates during operation and includes a generallycylindrical shell 36 driven by a motor 38. The rotating assembly's shell36 is disposed inside a stationary vapor-chamber housing 40 on which ismounted a gear housing 42 that additionally supports the motor 38. Thevapor-chamber housing 40 in turn rests in a support omitted from thedrawing for the sake of simplicity.

As FIG. 3's exemplary heat-exchanger plate 34 illustrates, each plate islargely annular; it may have an outer diameter of, say, 8.0 inches andan inner diameter of 3.35 inches. Each plate is provided with a numberof passage openings 46. FIG. 4, which is a cross section taken at line4—4 of FIG. 3, shows that the passage openings are formed with annularlips 48 that in alternating plates protrude upward and downward so that,as will explained in more detail presently, they mate to form passagesbetween the heat exchanger's condensation chambers.

To form alternating condensation and evaporation chambers, theheat-exchanger plates are provided with annular flanges 50 at theirradially inward edges and annular flanges 52 at their radially outwardedges. Like the passage lips 48, these flanges 50 and 52 protrude fromtheir respective plates, but in directions opposite those in which thepassage lips 48 protrude. FIG. 5, which depicts the radially inward partof the heat exchanger on the left and the radial outward part on theright, shows that successive plates thereby form enclosed condensationchambers 54 interspersed with open evaporation chambers 56. A recentlytested prototype of the heat exchanger employs 108 such plate pairs.

As will be explained in more detail below, a sprayer in the form of astationary spray arm 58 located centrally of the spinning heat-exchangerplates sprays water to be purified onto the plate surfaces that definethe evaporation chambers 56. (The use of the term spray is not intendedto imply that the water is necessarily or preferably applied indroplets, although some embodiments may so apply the liquid.) Thatliquid absorbs heat from those surfaces, and some of it evaporates. FIG.2's compressor 60 draws the resultant vapor inward.

FIG. 6 depicts compressor 60 in more detail. The compressor spins withthe rotary heat exchanger and includes a (spinning) compressor cylinder62 within which a mechanism not shown causes two pistons 64 and 66 toreciprocate out of phase with each other. As a piston rises, itsrespective piston ring 68 or 70 forms a seal between the piston and thecompressor cylinder 62's inner surface so that the piston draws vaporfrom the heat exchanger's central region. As a piston travels downward,on the other hand, its respective piston ring tends to lift off thepiston surface and thereby break the seal between the cylinder wall andthe pistons.

When their respective pistons are traveling downward, annularpiston-ring stops 72 and 74, which respective struts 76 and 77 secure torespective pistons 64 and 66, drag respective piston rings 68 and 70downward after the seal has been broken. The piston rings and stops thusleave clearances for vapor flow past the pistons as they move downward,so a downward-moving piston does not urge the vapor back downward aseffectively as an upward-moving piston draws it upward. Additionally,the pistons reciprocate so out of phase with each other that there isalways one piston moving upward, and thereby effectively drawing thevapor upward, while the other is returning downward.

As will be explained in more detail below, the vapor thus driven upwardby the pistons 64 and 66 cannot pass upward beyond the compressor'scylinder head 78, but slots 80 formed in the compressor wall's upper lipprovide paths by which the vapor thus drawn from the heat exchanger'scentral region can be driven down through an annular passage 82 formedbetween the compressor cylinder 62's outer surface and therotating-assembly shell 36. This passage leads to openings 83 in anannular cover plate 84 sealed by O-rings 85 a and 85 b between thecompressor cylinder 62 and the rotating-assembly shell 36. The openings83 register with the openings 46 (FIG. 3) that form the passages betweenthe condensation chambers.

In short, the compressor cylinder 62, the cylinder head 78, and therotating-assembly shell 36 cooperate to form a guide that directs vaporalong a vapor path from FIG. 5's evaporation chambers 56 to itscondensation chambers 54. And the compressor compresses the vapor thatfollows this path, so the vapor pressure in the condensation chambers 54is higher than that in the evaporation chambers 56, from which thecompressor draws the vapor. The boiling point in the condensationchambers therefore is also higher than in the evaporation chambers. Sothe heat of vaporization freed in the condensation chambers diffuses tothe (lower-temperature) evaporation chambers 56.

In the illustrated embodiment, the rotating assembly rotates at arelatively high rate of, say, 700 to 1000 rpm. The resultant centrifugalforce causes the now-purified condensate to collect in the outer ends ofthe condensation chambers, between which it can flow through thepassages that the heat-exchanger-plate openings 46 form. As FIG. 7shows, the condensate therefore flows out through the openings 83 in thetop of the heat exchanger and travels along the channel 82 by which thecompressed vapor flowed into the heat exchanger.

Like the compressed vapor, the condensate can flow through the openings80 in the compressor wall's lip. But the condensate can also flow pastthe cylinder head 78 because of a clearance 86 between that cylinderhead 78 and the rotating-assembly shell, whereas the condensate'spresence in that clearance prevents the compressed vapor from similarlyflowing past the cylinder head. An O-ring 88 seals between therotating-assembly shell 36 and a rotating annular channel-forming member90 secured to the cylinder head 78, but spaced-apart bosses 92 formed inthe cylinder head 78 provide clearance between the cylinder head and thechannel member so that the condensate, urged by the pressure differencethat the compressor imposes, can flow inward and into channel member90's interior.

Like the cylinder head 78 to which it is secured, the channel-formingmember 90 spins with the rotary heat exchanger to cause the purifiedcondensate that it contains to collect under the influence ofcentrifugal force in the channel's radially outward extremity. Thespinning condensate's kinetic energy drives it into a stationary scooptube 94, from which it flows to FIG. 1's condensate outlet 14 by way ofa route that will be described in due course.

While the scoop tube 94 is thus removing the liquid condensate that hasformed in the condensation chambers, centrifugal force drives theunevaporated feed liquid from the evaporation chambers to form anannular layer on the part of the rotating-assembly wall 36 below plate84: that wall thus forms a liquid-collecting sump. Another scoop tube,which will be described below, removes this unevaporated liquid forrecirculation through the rotary heat exchanger.

Before we deal with the manner in which the recirculation occurs, wesummarize the overall fluid circuit by reference to FIG. 8. A pump 100draws feed liquid from the feed inlet 12 and drives it to the cold-waterinlets 102 _(C) _(—) _(IN) and 104 _(C) _(—) _(IN) of respectivecounterflow-heat-exchanger modules 102 and 104. Those modules guide thefeedwater along respective feed-water paths to respective cold-wateroutlets 102 _(C) _(—) _(OUT) and 104 _(C) _(—) _(OUT). In flowing alongthose paths, the feedwater is in thermal communication with counterflowsthat enter those heat exchangers at hot-water inlets 102 _(H) _(—) _(IN)and 104 _(H) _(—) _(IN) and leave through hot-water outlets 102 _(H)_(—) _(OUT) and 104 _(H) _(—) _(OUT), as will be explained in moredetail below, so it is heated. (The terms hot and cold here respectivelyrefer to the fluid flows from which and to which heat is intended toflow in the counterflow heat exchangers. They are not intended to referto absolute temperatures; the liquid leaving a given counterflow heatexchanger's “cold”-water outlet, for instance, will ordinarily be hotterthan the liquid leaving its “hot”-water outlet.)

For reasons that will be set forth below, counterflow-heat-exchangermodule 104 receives a minor fraction of the feed-water flow driven bythe pump 100. Its volume flow rate is therefore relatively low, and thetemperature increase of which it is capable in a single pass isrelatively high as a consequence. For modularity purposes,counterflow-heat-exchanger module 102 in the illustrated embodiment isessentially identical to counterflow-heat-exchanger module 104, but itreceives a much higher volume flow rate, and the temperature increasethat it can impart is correspondingly low. So the cold-water flowthrough counterflow-heat-exchanger module 102 also flows seriallythrough further modules 106, 108, and 110 to achieve a temperatureincrease approximately equal to module 104's.

The series-connected modules' output from outlet 110 _(C) _(—) _(OUT) isfed to a degasser 112, as is the single heat exchanger 104's output fromoutlet 104 _(C) _(—) _(OUT). For the sake of simplicity, FIG. 2 omitsthe degasser, but the degasser would typically enclose the motor 38 toabsorb heat from it. The degasser thus further heats the liquid.Together with the heat imparted by the counterflow heat exchangers, thisheat may be enough to raise the feed-liquid temperature to the levelrequired for optimum evaporator/condenser action when steady-stateoperation is reached. From a cold start, though, a supplemental heatsource such as a heating coil (not shown) would in most cases contributeto the needed heat. The residence time in the degasser is long enough toremove most dissolved gasses and volatiles from the stream. Thethus-degassed liquid then flows to a filter assembly 114, where its flowthrough a filter body 116 results in particulate removal.

The resultant filtered liquid flows from the filter body 116 to anannular exit chamber 118, from which it issues in streams directed totwo destinations. Most of that liquid flows by way of tube 119 to anozzle 120. As FIG. 9 shows, nozzle 120 delivers the filtered feedliquid to the rotating-assembly shell 36's inner surface, where it joinsthe liquid layer formed by the liquid that has flowed through theevaporation chambers without evaporating. Only a minor fraction of theliquid that flows into the evaporation chambers evaporates in thosechambers in one pass, so most of it contributes to the rotating layer,whereas the feed nozzle 120 delivers only enough liquid to that layer toreplenish the fluid that has escaped by evaporation.

Stationary scoop tubes 122 and 124 scoop liquid from this rotatinglayer. The scooped liquid's kinetic energy drives it along those tubes,which FIG. 10 shows in plan view and FIGS. 11 and 12 show incross-sectional views respectively taken at lines 11—11 and 12—12 ofFIG. 10. To minimize the kinetic energy's dissipation, each scoop tubebends gradually to a predominantly radial direction. Also, each scooptube is relatively narrow at its entrance but widens gradually toconvert some of the liquid's dynamic head into static head. Those tubesguide the thus scooped liquid into an interior chamber 126 (FIG. 11) ofa transfer-valve assembly 128. Ordinarily, a transfer-valve member 130is oriented as FIG. 12 shows. In this orientation it permits flow fromthe interior chamber 126 through entry ports 132 into spray arms 58 butprevents flow through a port 134 into a conduit 136 that leads to anupper entrance of FIG. 8's filter assembly 114. The static head drivesthe liquid up the spray arms. FIG. 13, which is cross-sectional viewtaken at line 13—13 of FIG. 12, shows that each of the spray arms 58forms a longitudinal slit 138. These slits act as nozzles from which the(largely recirculated) liquid sprays into the evaporation chambers 56depicted in FIG. 5.

In short, the liquid-collecting inner surface of the rotating-assemblyshell 36, the scoop tubes 122 and 124, the transfer-valve assembly 128,and the spray arms 58 form a guide that directs unevaporated liquidalong a recirculation path that returns it to the evaporation chambers56. And, since FIG. 8's nozzle 120 supplements the recirculating liquidwith feed liquid, this guide cooperates with the main pump 100, thecounterflow heat exchangers 102, 104, 106, 108, and 110, the degasser112, the filter assembly 114, and the tubes that run between them aswell as tube 118 and nozzle 120 to form a further guide. This furtherguide directs feed liquid along a make-up path from the feed inlet 12 tothe evaporation chambers 56.

Now, so long as its evaporator-chamber surfaces stay wetted,heat-transfer efficiency in the rotary heat exchanger is greatest whenthe water film on these surfaces is thinnest. The flow volume throughthe spray arms 58 should therefore be so controlled as to leave thatfilm as thin as possible. In the illustrated embodiment, the flow ratethrough those spray arms is chosen to be just high enough to keep thesurfaces from drying completely between periodic wetting sprays from ascanner 140 best seen in FIG. 9. The scanner includes two scannernozzles 142 and 144 that provide a supplemental spray at two discrete(but changing) heights within the rotary heat exchanger.

The nozzles' heights change because a drive rod 146 reciprocates, in amanner that will presently be described in more detail, to raise andlower a yoke 148 from which the scanner 140 extends. Control of thescanner feed is best seen in FIG. 14, which is a cross-sectional view,with parts removed, of the vapor-chamber housing 40's lower interior.FIG. 14 depicts the valve member 130 in the closed state, but when thevalve member 130 is in its opposite, open state, it permits flow notonly into the spray tubes' ports 132 but also into a path through aseparate feed conduit 150 by way of an internal passage not shown into avertically extending tube 152. A telescoping conduit 154 that slides intube 152 conducts the flow, as best seen in FIG. 9, through the yoke 148and into the scanner 140. So these elements guide liquid along a furtherbranch of the recirculation and make-up paths.

As the reciprocating rod 146 drives the yoke 148 and thereby the scanner140 up and down, successive evaporation chambers momentarily receive asupplemental liquid spray. This spray is enough to wet the evaporatorsurfaces if they have become dry, or at least to prevent them fromdrying as they would if they were sprayed only through the spray arms58. The flow rate experienced by each of the evaporation chambers istherefore cyclical. The steady flow from the spray arms can be lowenough not to keep the surfaces wetted by itself. Indeed, the cyclicalspray can keep the surfaces wetted even if the average flow rate thatresults when the supplemental scanner spray is taken into account wouldnot be great enough to keep the surface wetted if it were appliedsteadily.

Under testing conditions that I have employed, for example, theirrigation rate required to keep the plates wetted is about 4.0gal./hr./plate if the irrigation rate is kept constant. But I have beenable to keep the heat-transfer surfaces wetted when the spray armstogether sprayed 216 gal./hr. on 216 plates, or only 1.0 gal/hr./plate.True, this spray was supplemented by the spray from the scanner. But thescanner nozzles together contributed only 30 gal./hr. Since the scannernozzles together overlap two evaporation chambers in my prototype so asto spray an average of four plates at a time, this meant that thescanner sprayed each plate for about 4/216=1.9% of the time at about 30gal./hr.÷4 plates=7.5 gal./hr./plate. Although the resultant peakirrigation rate was therefore 8.5 gal./hr./plate, which exceeds theconstant rate required to keep the plates wetted, the average irrigationrate was only 1.14 gal./hr./plate, or only 28% of that constant rate of4.0 gal./hr./plate. Such a low rate contributes to heat-exchangerefficiency, because it permits the average film thickness to be madeless without drying than would be possible with only a steady spray.While it is not necessary to use these particular irrigation rates, mostembodiments of the present invention will employ average rates no morethan half the constant rate required for wetting, while the peak ratewill exceed that constant rate.

The manner in which the scanner 140's reciprocation is provided is notcritical to the present invention; those skilled in the art willrecognize many ways in which to cause reciprocation. But the way inwhich the illustrated embodiment provides the reciprocation isbeneficial because it takes advantage of the mechanisms used to refreshthe rotary-heat-exchanger fluid and to back flush the filter. Tounderstand those mechanisms, it helps to refer to FIG. 14.

FIG. 14 shows that the transfer-valve assembly 128 is provided on avapor-chamber base 160 sealingly secured to the vapor-chamber housing40's lower annular lip 162. Together that lip and the vapor-chamber basecan be thought of as forming a secondary, stationary sump that catchesany spillage from the main, rotating sump. The heating coil mentionedabove for use on startup may be located in that sump and raise thesystem to temperature by heating sump liquid whose resultant vaporcarries the heat to the remainder of the system.

Among the several features that the vapor-chamber base 160 forms is avertical transfer-pump port 164, through which the drive rod 146extends. That rod extends into a transfer pump 166 that FIG. 14 omitsbut FIG. 15 illustrates in cross section. The transfer pump 166 includesan upper cylinder half 168 that forms a cylindrical lip 169, which mateswith the transfer-pump port 164 of FIG. 14. It also forms a flange 170by which a bolt 172 secures it to a corresponding flange 174 formed on alower cylinder half 176. FIG. 15 also depicts a mounting post 178, whichis one of two that are secured to FIG. 14's vapor-chamber base 160 andsupport the transfer pump 166 by means of flanges, such as flange 180,formed on the upper cylinder half 168.

A piston 182 is movably disposed inside the transfer-pump cylinder thathalves 168 and 176 form, and a spring 184 biases the piston 182 into theposition that FIG. 15 depicts. As that drawing illustrates, the driverod 146 is so secured to the piston 182 as to be driven by it as thepiston reciprocates in response to spring 184 and fluid flows that willnow be described by reference to FIG. 8.

It will be recalled that the filter assembly 114's output is dividedbetween two flows. In addition to the liquid-make-up flow through tube119 to the feed nozzle 120, there is a second, smaller flow throughanother tube 186. This tube leads to a channel, not shown in FIG. 14,that communicates with an upper section 188, which FIG. 14 does show, ofthe transfer-pump port 164. During most of its operating cycle, thepiston 182 shown in FIG. 15 moves slowly downward in response to theforce of its bias spring 184 and thereby draws liquid from FIG. 8's tube186 through port 164 into the portion of the transfer pump's interiorabove the piston 182. As will be seen, this portion serves as arefresh-liquid reservoir, and the components that guide feed liquid fromFIG. 8's feed inlet 12 through the filter assembly 114 cooperate withtube 186 and port 164 to form a guide that directs feed liquid along afeed-liquid-storage path into that reservoir.

As will also be seen, the pump's lower portion serves as a concentratereservoir. While the piston is drawing liquid into the refresh-liquidreservoir, it is expelling liquid from the concentrate reservoir throughan output port 190 formed, as FIG. 15 shows, by the lower cylinder half176. The lower cylinder half further forms a manifold 192. One outlet194 of that manifold leads to the filter assembly 114, which FIG. 15omits but FIG. 16 depicts in cross section. FIG. 16 shows that thefilter assembly includes a check valve 196 that prevents flow into thefilter assembly from manifold outlet 194. As FIG. 15 shows, the flowleaving the transfer pump from its lower outlet 190 must therefore flowthrough the other manifold outlet 198.

FIG. 8 shows that a tube 200 receives that transfer-pump output. A flowrestricter 202 in that tube limits its flow and thus the rate at whichthe transfer-pump piston can descend. By thus limiting the transfer-pumppiston 182's rate of descent, flow restricter 202 also limits how muchof the filter assembly 114's output flows through tube 186 into thetransfer pump 166's upper side, with the result that the transfer pumpreceives only a small fraction of the filter output and thus of theoutput from the input pump 100. A flow divider comprising a flowjunction 203 and another flow restricter 204 so controls the proportionof pump 100's output that feeds counterflow-heat-exchanger module 104'scold side that this cold-side flow approximates the hot-side flow thatflow restricter 202 permits: main pump 100's output is divided in thesame proportion as the transfer pump 166's output is. As was mentionedabove, the resultant relatively low flow rate into module 104 is whatenables the entire heat transfer to occur in a single module 104,whereas the higher flow rate through modules 102, 106, 108, and 110necessitates, their series combination.

Because of the flow restricter 202, FIG. 15's transfer-pump piston 182moves downward under spring force at a relatively leisurely rate,taking, say, five minutes to proceed from the top to the bottom of thetransfer-pump cylinder. As the piston descends, it draws the drive rod146 downward with it, thereby causing FIG. 9's scanner nozzles 142 and144 to scan respective halves of the rotary heat exchanger's set ofevaporation chambers. At the same time, it slides an actuator sleeve 206provided by yoke 148 along an actuator rod 208.

As FIG. 17 shows, a spring mount 210 is rigidly secured to the actuatorrod 208 and so mounts a valve-actuating spring 212 that the spring's tipfits in the crotch 214 of a valve crank 216. The spring engages thecrank in an over-center configuration that ordinarily keeps thatactuator rod 208 in the illustrated relatively elevated position. Thevalve crank 216 is pivotably mounted in the transfer-valve assembly andsecured to FIG. 12's transfer-valve member 130 to control its state.

When the valve crank 216 is in its normal, upper position depicted inFIG. 17, the transfer-valve member 130 is in the lower position,depicted in FIG. 12, in which it directs liquid from the scoop tubes 122and 124 (FIG. 10) to flow into the spray arms 58 and scanner 140 but notinto the filter inlet port 134. As FIG. 9's yoke 148 continues itsdescent, though, its actuator sleeve 206 eventually begins to bearagainst a buffer spring 218 that rests on the spring mount 210's upperend. The resultant force on the mount and thus on the actuator rod 208overcomes the restraining force of FIG. 17's valve-actuating spring 212,causing the valve crank 216 to snap to its lower position. It therebyoperates FIG. 12's valve member 130 from its position illustrated inFIG. 12 to its FIG. 18 position, in which it redirects the scoop-tubeflow from the spray arms 58 to the conduit 136 that feeds the filterassembly's upper inlet 220 (FIG. 16).

Now, whereas fluid ordinarily flows through the filter at only therelatively low rate required to compensate for evaporation, the flowdirected by this transfer-valve actuation into the filter is the entirerecirculation flow; that is, it includes all of the liquid that hasflowed through FIG. 5's evaporation chambers 56 without evaporating.Since only a relatively small proportion of the liquid that is fed tothe evaporation chambers actually evaporates in any given pass, therecirculation flow is many times the feed flow, typically twenty times.

The pressure that this high flow causes within the filter assembly opensthe filter assembly's check valve 196 (FIG. 16) and thereby permits therecirculation flow to back through the outlet 194 of FIG. 15'stransfer-pump-output manifold 192 and, because of the resistance offeredby flow restricter 202 (FIG. 8), back through the transfer pump's outlet190 to the concentrate reservoir. With the transfer valve in this state,that is, the scoop tubes 122 and 124 (FIG. 10), the transfer-valveassembly 128, and the filter assembly 114 (FIG. 16) form a guide thatdirects concentrate from the liquid-collecting inner surface of therotating-assembly shell 36 (FIG. 9) along a concentrate-storage path tothe transfer pump's concentrate reservoir.

That redirected flow flushes the filter so as to reduce its impuritiesload and thus the maintenance frequency it would otherwise require. Italso drives the transfer-pump piston 182 (FIG. 15) rapidly upward. Thepiston in turn rapidly drives the feed liquid that had slowlyaccumulated in the transfer pump's upper, refresh-reservoir portion outthrough the vapor-chamber base's port 164 (FIG. 14) along a refreshpath. As FIG. 14 shows, that is, it flows into ports 132 by way of acheck valve 224 provided to prevent recirculation flow from entering therefresh reservoir. With that flow now redirected to the transfer pump'slower side, i.e., to the concentrate reservoir, the resultant rapid flowthrough the check valve 224 and ports 132 enters the spray arms 58 andscanner 140, replacing the temporarily redirected recirculation flow.All this happens in a very short fraction of the recirculation cycle. Inmost embodiments, the duration of this refresh cycle will be only on theorder of about a second, in contrast to the recirculation cycle, whichwill preferably be at least fifty times as long, typically lastingsomewhere in the range of two to ten minutes.

The effect of thus redirecting the feed and recirculation flows is toreplace the rotary heat exchanger's liquid inventory with feed liquidthat has not recirculated. As was explained previously, the rotary heatexchanger continuously removes vapor from the evaporation side, leavingimpurities behind and sending the vapor to the condensation side. Soimpurities tend to concentrate in the recirculation flow. Suchimpurities may tend to deposit themselves on the heat-exchange surfaces.Although the periodic surface flushing that the scanner nozzles performgreatly reduces this tendency, it is still desirable to limit theimpurities concentration. One could reduce impurities in a continuousfashion, continuously bleeding off some of the recirculation flow asconcentrate exhaust. But the illustrated embodiment periodicallyreplaces essentially the entire liquid inventory on the rotary heatexchanger's evaporation side. This results in an evaporator-sideconcentration that can average little more than half the exhaustconcentration. So less water needs to be wasted, because the exhaustconcentration can be higher for a given level of tolerated concentrationin the system's evaporator side.

As the transfer-pump piston rises rapidly, it slides FIG. 9's actuatorsleeve 206 upward rapidly, too. Eventually, the sleeve begins tocompress a further buffer spring 226 against a stop 230 that theactuator rod 208 provides at its upper end. At some point, the resultantupward force on the actuator rod 208 overcomes the restraining forcethat FIG. 17's valve-actuating spring 212 exerts on it through thespring mount 210, and the actuator rod rises to flip the valve crank 216back to its upper position and thus return the transfer valve 130 to itsnormal position, in which the recirculation flow from FIG. 9's scooptubes 122 and 124 is again directed to the spray arms and scanner. Sothe unit returns to its normal regime, in which the transfer pump slowlyexpels concentrate from its concentrate reservoir and draws feed liquidthrough the feed-liquid storage path to its refresh-liquid reservoir. AsFIG. 8 shows, tube 200, counterflow-heat-exchanger module 104, and afurther tube 232 guide the concentrate thus expelled along aconcentrate-discharge path from manifold outlet 198 to the concentrateoutlet 16.

To achieve approximately the same peak concentration in differentinstallations despite differences in those installations' feed-liquidimpurity levels, different refresh-cycle frequencies may be used indifferent installations. And, since the typical feed-liquid impuritylevel at a given installation may not always be known before the unit isinstalled—or at least until rather late in the distiller's assemblyprocess—some embodiments may be designed to make that frequencyadjustable.

For example, some embodiments may make the piston travel adjustable by,for instance, making the position of a component such as FIG. 9's stop230 adjustable. In the illustrated embodiment, though, that travel alsocontrols scanner travel, and any travel adjustability would instead beused to obtain proper scanner coverage. So one may instead affectfrequency by adjusting the force of FIG. 15's transfer-pump spring 184.This could be done by, for instance, making the piston 182's position onthe drive rod 146 adjustable. Refresh-frequency adjustability could alsobe provided by making the flow resistance of FIG. 8's flow restricter202 adjustable.

In any case, flow restricter 204, which balances the twocounterflow-heat-exchanger flows to match the relative rate ofconcentrate discharge, would typically also be made adjustable if therefresh-cycle frequency is. The flow restricters could take the form ofadjustable bleed valves, for instance.

Having now described the distillation unit's rotary heat exchanger, wewill describe one of its counterflow-heat-exchanger modules. Beforedoing so, though, we return to FIG. 8 to complete the discussion of thefluid circuit in which those modules reside. The flow of purified liquidthat issues from FIG. 7's condensate scoop tube 94 is directed to FIG.8's accumulator 236, which the drawings do not otherwise show. Theaccumulator 236 receives condensate in a resiliently expandable chamber.The accumulator's output feeds heat-exchanger module 110's hot-waterinlet 110 _(H) _(—) _(IN) to provide the hot-side flow through theserial combination of heat exchangers 110, 108, 106, and 102. Acondensate pump 238 drives this flow. After being cooled by flow throughthe serial heat-exchanger-module combination, the cooled condensateissues from module 102's “hot”-water outlet 102 _(H) _(—) _(OUT) andflows through a pressure-maintenance valve 240 and the concentrateoutlet 16. Valve 240 keeps the pressure in the hot sides of counterflowheat exchangers 102, 106, 108, and 110 higher than in their cold sidesso that any leakage results in flow from the pure-water side to thedirty-water side and not vice versa.

The main pump 100's drive is controlled in response to a pressure sensor242, which monitors the rotary heat exchanger's evaporator-side pressureat some convenient point, such as the transfer valve's interior chamber.Finally, to accommodate various leakages, tubes to the drain outlet 18may be provided from elements such as the pump, pressure-maintenancevalve, and sump.

It can be seen from the description so far that thecounterflow-heat-exchanger modules 102, 104, 106, and 108 act as atemperature-transition section. The rotary-heat-exchanger part of thefluid circuit is a distiller by itself, but one that relies on ahigh-temperature input and produces high-temperature outputs. Thecounterflow-heat-exchanger modules make the transition between thosehigh temperatures and the relatively low temperatures at the feed inletand condensate and concentrate outlets. The counterflow-heat-exchangermodules in essence form two heat exchangers, which respectively transferheat from the condensate and concentrate to the feed liquid. We now turnto one example of the simple type of counterflow-heat-exchanger modulethat this arrangement permits.

FIG. 19, which is an isometric view of counterflow heat exchanger 102with parts removed, shows tubes that provide its cold-water inlets 102_(C) _(—) _(IN) and 102 _(C) _(—) _(OUT). It also shows the hot-wateroutlet 102 _(H) _(—) _(OUT) but not the hot-water inlet, which ishidden. FIG. 20 is a cross section taken through the cold-water inlet102 _(C) _(—) _(IN) and the hot-water outlet 102 _(H) _(—) _(OUT). Thatdrawing shows that heat exchanger 102 includes a generally U-shapedchannel member 250, which provides an opening 252 that communicates withthe heat exchanger's “hot”-side outlet. Similar openings 254 in a cover258 and gasket 260 (both of which FIG. 19 omits) provide the cold-waterinlet 102 _(C) _(—) _(IN). A folded stainless-steel heat-transfer sheet262 provides the heat-exchange surfaces that divide the cold-water sidefrom the hot-water side, and elongated clips 264 secure the foldedsheet's flanges 266, channel-member flanges 268, cover 258, and covergasket 260.

As FIG. 19 shows, spacer combs 270 are provided at spaced-apartlocations along the heat exchanger's length. One spacer comb 270's teeth272 are visible in FIG. 20, and it can be seen that the teeth help tomaintain proper bend locations in the folded heat-transfer sheet 262.Similar teeth 274 of a similar spacer comb at the opposite side of theheat-transfer sheet 262 also serve to space its bends.

FIG. 19 shows the upper surfaces of diverter gaskets 278, which extendbetween the upper spacer combs 270 and serve to restrict the cold-waterflow to regions close to the folded heat-transfer sheet 262's uppersurface. FIG. 19 also shows that the module includes end plates 280 and281. These end plates cooperate with the channel member 250, the cover258, and the cover gasket 260 to form a closed chamber divided by thesheet 262. Additionally, the leftmost diverter gasket 278 cooperateswith the end plate 280 and the cover 258 and cover gasket 260 to form aplenum 282 (FIG. 20) by which cold water that has entered through port102 _(C) _(—) _(IN) is distributed among the heat-exchange-surface sheet262's several folds.

End plate 280 similarly cooperates with another diverter gasket 284(FIG. 20) to form a similar plenum 286 by which water on the hot-waterside that has flowed longitudinally along the heat-exchange surfacesissues from the heat exchanger 102 by way of its hot-water outlet 102_(H) _(—) _(OUT). Incoming hot-side water and outgoing cold-side waterflow through similar plenums at the other end.

It can be appreciated from the foregoing description that the presentinvention's teachings can significantly increase anevaporator-and-condenser unit's operating efficiency. It thusconstitutes a significant advance in the art.

What is claimed is:
 1. For distilling a liquid, anevaporator-and-condenser unit comprising: a heat exchanger that forms atleast one condensation chamber and a plurality of evaporation chambersand includes heat-transfer surfaces by which heat passes from the atleast one condensation chamber to the plurality of evaporation chambers;a varying-rate evaporation-chamber irrigation system whose rate ofirrigation of each of the evaporation chambers has a respective averageirrigation rate and so varies as repeatedly to reach a respective peakirrigation rate that is at least twice the average irrigation ratethereof, wherein the times at which the rates of irrigation of some ofthe evaporation chambers reach their respective peak irrigation ratesare different from those at which others of the plurality of evaporationchambers do; and a vapor guide defining a vapor path along which itdirects to the at least one condensation chamber vapor thereby producedin the plurality of evaporation chambers, wherein the irrigation systemfurther includes: A) a main sprayer system, which irrigates each saidevaporation chamber for at least the majority of the time; and B) anauxiliary sprayer system, which irrigates each of the evaporationchambers for only a minority of the time, the rate at which each saidevaporation chamber is irrigated while the auxiliary sprayer system isirrigating it being at least twice the average irrigation rate thereof.2. An evaporator-and-condenser unit as defined in claim 1 wherein theauxiliary sprayer system includes a plurality of auxiliary-systemnozzles from which the auxiliary sprayer system produces anauxiliary-system spray by which the auxiliary sprayer system irrigatesthe evaporation chambers.
 3. An evaporator-and-condenser unit as definedin claim 1 wherein the main sprayer system includes a plurality ofmain-system nozzles from which the main sprayer system produces amain-system spray by which the main sprayer system irrigates theevaporation chambers.
 4. An evaporator-and-condenser unit as defined inclaim 1 further including a compressor so interposed in the vapor pathas to make the vapor pressure in the at least one condensation chamberexceed that in the evaporation chambers.
 5. For distilling a liquid, anevaporator-and-condenser unit comprising: A) a heat exchanger that formsat least one condensation chamber and at least one evaporation chamberand includes heat-transfer surfaces by which heat passes from the atleast one condensation chamber to the at least one evaporation chamber;B) a varying-rate evaporation-chamber irrigation system whose rate ofirrigation of each said evaporation chamber has a respective averageirrigation rate and so varies as repeatedly to reach a respective peakirrigation rate that is at least twice the average irrigation ratethereof, wherein the peak irritation rate for each said at least oneevaporation chamber exceeds the steady-state rate required to keep theheat-transfer surfaces thereof wetted, and the average irrigation ratefor each said at least one evaporation chamber is no more than half thesteady-state rate required to keep the heat-transfer surfaces of thatevaporation chamber wetted; and C) a vapor guide defining a vapor pathalone which it directs to the at least one condensation chamber vaporthereby produced in the at least one evaporation chamber; and D) acompressor so interposed in the vapor path as to make the vapor pressurein the at least one condensation chamber exceed that in the at least oneevaporation chamber.
 6. For distilling a liquid, anevaporator-and-condenser unit comprising: a heat exchanger that forms atleast one condensation chamber and a plurality of evaporation chambersand includes heat-transfer surfaces by which heat passes from the atleast one condensation chamber to the plurality of evaporation chambers:a varying-rate evaporation-chamber irrigation system whose rate ofirrigation of each said plurality of evaporation chambers has arespective average irrigation rate and so varies as repeatedly to reacha respective peak irrigation rate that is at least twice the averageirrigation rate thereof, wherein the peak irrigation rate for each saidplurality of evaporation chambers exceeds the steady-state rate requiredto keep the heat-transfer surfaces thereof wetted, and the averageirrigation rate for each said plurality of evaporation chambers is nomore than half the steady-state rate required to keep the heat-transfersurfaces of that evaporation chamber wetted, and wherein the times atwhich the rates of irrigation of some of the evaporation chambers reachtheir respective peak irrigation rates are different from those at whichothers of the evaporation chambers do; and a vapor guide defining avapor path along which it directs to the at least one condensationchamber vapor thereby produced in the plurality of evaporation chambers;wherein the irrigation system further includes: A) a main sprayersystem, which irrigates each said evaporation chamber for at least themajority of the time; and B) an auxiliary sprayer system, whichirrigates each said at least one evaporation chamber for only a minorityof the time, the rate at which each said evaporation chamber isirrigated while the auxiliary sprayer system is irrigating it being atleast twice the average irrigation rate thereof.
 7. For distilling aliquid, an evaporator-and-condenser unit comprising: A) a heat exchangerthat forms at least one condensation chamber and at least oneevaporation chamber and includes heat-transfer surfaces by which heatpasses from the at least one condensation chamber to the at least oneevaporation chamber; B) a varying-rate evaporation-chamber irrigationsystem whose rate of irrigation of each said evaporation chamber has arespective average irrigation rate and so varies as repeatedly to reacha respective peak irrigation rate that is at least twice the averageirrigation rate thereof, wherein the peak irrigation rate for each saidat least one evaporation chamber exceeds the steady-state rate requiredto keep the heat-transfer surfaces thereof wetted, and the averageirrigation rate for each said at least one evaporation chamber is nomore than half the steady-state rate required to keep the heat-transfersurfaces of that evaporation chamber wetted; and C) a vapor guidedefining a vapor path along which it directs to the at least onecondensation chamber vapor thereby produced in the at least oneevaporation chamber, wherein the heat exchanger is a rotary heatexchanger in which the heat-transfer surfaces are mounted for rotationabout a central cavity from which the irrigation system irrigates the atleast one evaporation chamber.
 8. An evaporator-and-condenser unit asdefined in claim 7 further including a compressor so interposed in thevapor path as to make the vapor pressure in the at least onecondensation chamber exceed that in the at least one evaporationchamber.
 9. An evaporator-and-condenser unit as defined in claim 7wherein: A) the evaporation-and-condenser unit includes a plurality ofthe evaporation chambers; and B) the times at which the rates ofirrigation of some of the evaporation chambers reach their respectivepeak irrigation rates are different from those at which others of theevaporation chambers do.
 10. A method as defined in claim 9 wherein eachevaporation chamber's irrigation rate reaches its peak irrigation rateperiodically.
 11. An evaporator-and-condenser unit as defined in claim 9wherein the irrigation system includes: A) a main sprayer system, whichirrigates each said evaporation chamber for at least the majority of thetime; and B) an auxiliary sprayer system, which irrigates each said atleast one evaporation chamber for only a minority of the time, the rateat which each said evaporation chamber is irrigated while the auxiliarysprayer system is irrigating it being at least twice the averageirrigation rate thereof.
 12. An evaporator-and-condenser unit as definedin claim 11 further including a compressor so interposed in the vaporpath as to make the vapor pressure in the at least one condensationchamber exceed that in the at least one evaporation chamber.
 13. Anevaporator-and-condenser unit as defined in claim 11 wherein: A) theevaporator-and-condenser unit includes a plurality of said evaporationchambers; B) the auxiliary sprayer system includes at least oneauxiliary-system nozzle, associated with at least some of saidevaporation chambers, from which the auxiliary sprayer system producesan auxiliary-system spray; and C) for each of the evaporation chamberswith which the auxiliary-system nozzle is associated, theauxiliary-system nozzle executes reciprocation between positions inwhich the auxiliary system spray irrigates that evaporation chamber andpositions in which the auxiliary-system spray does not irrigate thatevaporation chamber.
 14. An evaporator-and-condenser unit as defined inclaim 13 further including a compressor so interposed in the vapor pathas to make the vapor pressure in the at least one condensation chamberexceed that in the at least one evaporation chamber.
 15. For distillinga liquid, an evaporator-and-condenser unit comprising: A) a heatexchanger that forms at least one condensation chamber and at least oneevaporation chamber and includes heat-transfer surfaces by which heatpasses from the at least one condensation chamber to the at least oneevaporation chamber; B) means for irrigating each said evaporationchamber at an irrigation rate that has a respective average irrigationrate and so varies as repeatedly to reach a respective peak irrigationrate that is at least twice the average irrigation rate thereof; and C)a vapor guide defining a vapor path along which it directs to the atleast one condensation chamber vapor thereby produced in the at leastone evaporation chamber.
 16. For distilling a liquid, anevaporator-and-condenser unit comprising: A) a heat exchanger that formsat least one condensation chamber and a plurality of evaporationchambers and includes heat-transfer surfaces by which heat passes fromthe at least one condensation chamber to the evaporation chambers; B) avarying-rate evaporation-chamber irrigation system whose rate ofirrigation of each said evaporation chamber has a respective averageirrigation rate and so varies as repeatedly to reach a respective peakirrigation rate that is at least twice the average irrigation ratethereof, the times at which at least one of the evaporation chambersreaches its peak irrigation rate differing from the times at which atleast one other of the evaporation chambers does, the irrigation systemincluding: i) a main sprayer system, which irrigates each saidevaporation chamber for at least the majority of the time; and ii) anauxiliary sprayer system, which irrigates each said at least oneevaporation chamber for only a minority of the time and includes atleast one auxiliary-system nozzle, associated with at least some of saidevaporation chambers for each of which that auxiliary-system nozzleexecutes reciprocation between positions in which the auxiliary-systemspray irrigates that evaporation chamber and positions in which theauxiliary-system spray does not irrigate that evaporation chamber, therate at which each said evaporation chamber is irrigated while theauxiliary sprayer system s irrigating it being at least twice theaverage irrigation rate thereof; and C) a vapor guide defining a vaporpath along which it directs to the at least one condensation chambervapor thereby produced in the plurality of evaporation chambers.
 17. Anevaporator-and-condenser unit as defined in claim 16 further including acompressor so interposed in the vapor path as to make the vapor pressurein the at least one condensation chamber exceed that in the evaporationchambers.
 18. For distilling a liquid, an evaporator-and-condenser unitcomprising: A) a heat exchanger that forms at least one condensationchamber and a plurality of evaporation chambers and includesheat-transfer surfaces by which heat passes from the at least onecondensation chamber to the plurality of evaporation chambers; B) avarying-rate evaporation-chamber irrigation system whose rate ofirrigation of each of the evaporation chambers has a respective averageirrigation rate and so varies as repeatedly to reach a respective peakirrigation rate that is at least twice the average irrigation ratethereof, the times at which at least one of the evaporation chambersreaches its peak irrigation rate differing from the times at which atleast one other of the evaporation chambers does, the irrigation systemincluding: i) a main sprayer system, which irrigates each of theevaporation chambers for at least the majority of the time; and ii) anauxiliary sprayer system that irrigates each of the evaporation chambersfor only a minority of the time, the rate at which each of theevaporation chambers is irrigated while the auxiliary sprayer system isirrigating it being at least twice the average irrigation rate thereof;and C) a vapor guide defining a vapor path along which it directs to theat least one condensation chamber vapor thereby produced in theevaporation chambers.
 19. An evaporator-and-condenser unit as defined inclaim 18 wherein the heat exchanger is a rotary heat exchanger in whichthe heat-transfer surfaces are mounted for rotation about a centralcavity from which the irrigation system irrigates the plurality ofevaporation chambers.
 20. An evaporator-and-condenser unit as defined inclaim 19 wherein: A) the evaporation-and-condenser unit includes aplurality of the evaporation chambers; and B) the times at which therates of irrigation of some of the evaporation chambers reach theirrespective peak irrigation rates are different from those at whichothers of the evaporation chambers do.
 21. An evaporator-and-condenserunit as defined in claim 19 wherein each evaporation chamber'sirrigation rate reaches its peak irrigation rate periodically.
 22. Anevaporator-and-condenser unit as defined in claim 19 further including acompressor so interposed in the vapor path as to make the vapor pressurein the at least one condensation chamber exceed that in the evaporationchambers.
 23. An evaporator-and-condenser unit as defined in claim 18further including a compressor so interposed in the vapor path as tomake the vapor pressure in the at least one condensation chamber exceedthat in the evaporation chambers.
 24. An evaporator-and-condenser unitas defined in claim 18 wherein: A) the auxiliary sprayer system includesat least one auxiliary-system nozzle, associated with at least some ofsaid evaporation chambers, from which the auxiliary sprayer systemproduces an auxiliary-system spray; and B) for each of the evaporationchambers with which the auxiliary-system nozzle is associated, theauxiliary-system nozzle executes reciprocation between positions inwhich the auxiliary-system spray irrigates that evaporation chamber andpositions in which the auxiliary-system spray does not irrigate thatevaporation chamber.
 25. An evaporator-and-condenser unit as defined inclaim 24 further including a compressor so interposed in the vapor pathas to make the vapor pressure in the at least one condensation chamberexceed that in the evaporation chambers.
 26. For distilling a liquid, anevaporator-and-condenser unit comprising: A) a heat exchanger that formsat least one condensation chamber and a plurality of evaporationchambers and includes heat-transfer surfaces by which heat passes frontthe at least one condensation chamber to the evaporation chambers; B) avarying-rate evaporation-chamber irrigation system whose rate ofirrigation of each said evaporation chamber has a respective averageirrigation rate and so varies as repeatedly to reach a respective peakirrigation rate that is at least twice the average irrigation ratethereof, the times at which at least one of the evaporation chambersreaches its peak irrigation rate differing from the times at which atleast one other of the evaporation chambers does, the evaporationchambers' peak irrigation rates exceeding the steady-state rate requiredto keep the heat-transfer surfaces thereof wetted, but the evaporationchambers' average irrigation rates being no more than half thatsteady-state rate, the irrigation system including: i) a main sprayersystem, which irrigates each of the evaporation chambers for at leastthe majority of the time; and ii) an auxiliary sprayer system, whichirrigates each of the evaporation chambers for only a minority of thetime, the rate at which each of the evaporation chambers is irrigatedwhile the auxiliary sprayer system is irrigating it being at least twicethe average irrigation rate thereof; C) a vapor guide defining a vaporpath along which it directs to the at least one condensation chambervapor thereby produced in the at least one evaporation chamber; and D) acompressor so interposed in the vapor path as to make the vapor pressurein the at least one condensation chamber exceed that in the evaporationchambers.
 27. For distilling a liquid, an evaporator-and-condenser unitcomprising: A) a heat exchanger that forms at least one condensationchamber and a plurality of evaporation chambers and includesheat-transfer surfaces by which heat passes from the at least onecondensation chamber to the evaporation chambers; B) a varying-rateevaporation-chamber irrigation system whose rate of irrigation of eachsaid evaporation chamber has a respective average irrigation rate and sovaries as repeatedly to reach a respective peak irrigation rate that isat least twice the average irrigation rate thereof, the times at whichat least one of the evaporation chambers reaches its peak irrigationrate differing from the times at which at least one other of theevaporation chambers does, the evaporation chambers' peak irrigationrates exceeding the steady-state rate required to keep the heat-transfersurfaces thereof wetted, but the evaporation chambers' averageirrigation rates being no more than half that steady-state rate, theirrigation system including: i) a main sprayer system, which irrigateseach of the evaporation chambers for at least the majority of the time;and ii) an auxiliary sprayer system, which irrigates each of theevaporation chambers for only a minority of the time and includes atleast one auxiliary-system nozzle, associated with at least some of saidevaporation chambers for each of which that auxiliary-system nozzleexecutes reciprocation between positions in which the auxiliary systemspray irrigates that evaporation chamber and positions in which theauxiliary-system spray does not irrigate that evaporation chamber, therate at which each of the evaporation chambers is irrigated while theauxiliary sprayer system is irrigating it being at least twice theaverage irrigation rate thereof; and C) a vapor guide defining a vaporpath along which it directs to the at least one condensation chambervapor thereby produced in the evaporation chambers.
 28. Anevaporator-and-condenser unit as defined in claim 27 further including acompressor so interposed in the vapor path as to make the vapor pressurein the at least one condensation chamber exceed that in the evaporationchambers.